1:45 PM - 3:15 PM
[O11-P89] Comparison of reef flat width and fossil coral colony size in Holocene terrace at Kikai Island, Japan
Keywords:coral reef, Holocene epoch
1. Introduction
Kikai Island, Kagoshima Prefecture, is an uplifted coral reef plain that has become land along the sea. Four terrace planes (planes I-IV in order from the inland side) can be seen surrounding the island, which were formed by uplift during the Holocene. Aerial photographs show that the size of the reef fields differs from coast to coast. We began this study to investigate what factors are responsible for the differences in the size of the reef fields.
Methodology
Based on aerial photographs, we investigated the Holocene terrace III (formed between 4100 and 3100 years ago during the last 1000 years) at Onotsu, Shidoake, and Hanaraji, which are located on the east and west sides of the islands of different sizes. We set up a measuring line perpendicular to the coastline on Plane III and recorded the distance of Plane III. The number, size, type, and location of fossil coral assemblages found along the survey line were recorded. We also used a simple laser rangefinder to measure the distance and height difference between each point according to the terrace topography. The seafloor topography was taken from "Minna no Nautical Chart" and compared with the topographic map of the reef field.
Results
The topography of the terraces on the surface III was gentle in the order of Shido Oke, Hanaraji, and Onotsu. The size of the reef field was 50 m at Shido Oke, 43.5 m at Hanaraji, and 8 m at Onotsu with respect to the size of the Holocene terrace III (Figure 1). Compared to the present seafloor topography, the slope of the topography was more gentle at Shido Oke, Hanaraji, and Onotsu, in that order. The average sizes of the fossil coral assemblages at each site were 35.9 cm, 27.9 cm, and 16.7 cm in length at Shido Oke, Hanararo, and Onotsu, respectively. The size of the III surfaces was wider and larger corals were found in the order of the slope of the terrain being more gradual. The coral species were examined at Hanaraji, and out of the 86 colonies on the surveyed line, 67 were of the genus Green Lampreys and 13 were of the family Chrysophyllidae. Of the genus Greenishi, most of the species that could be identified were Yasuri Greenishi and Nioidori Greenishi. The frequency of green lanceolate and yellow lanceolate by length diameter showed that corals between 10 and 19 cm were the most frequent in both genera, and the overall frequency distribution was similar. As for the present seafloor topography, the slope of the seafloor topography and the slope of the reef plain tended to be the same.
4. Discussion
Reefs with a high percentage of corals with large diameter were wider than those with small diameter, suggesting a relationship between the diameter of corals and the size of the reefs. The similarity in the frequency of different coral species by diameter suggests that there is no difference in size distribution among coral species. The slope of both the topography and seafloor topography of the reef field was gentle in the wide area of the reef field and steep in the narrow area, suggesting that the slope of the seafloor topography is significantly related to the size of the reef field. The present bathymetry shows that the slopes of Shidoake and Hanaraji are more gradual, and therefore, it is expected that the reef field will expand more easily in the future.
5. Conclusion and Prospects
From the present experiment, it is considered that the size of the reef field is related to the seafloor topography and the size of the coral length. Considering the habit of corals to grow horizontally instead of vertically at depths shallower than 5 m, corals are expected to grow horizontally at shallower depths, and therefore, the seafloor topography is considered to be particularly relevant. In this presentation, we will also calculate and present the Coral Carbonate Production (CCP), which compares the amount of CO2 fixed by each reef plain.
In the future, we would like to increase the number of survey sites in order to confirm the relationship between the reef fields, corals, and seafloor topography that we have learned in this study. In addition to topography, we would also like to explore other factors such as the effect of wind, which we could not consider in this study.
Kikai Island, Kagoshima Prefecture, is an uplifted coral reef plain that has become land along the sea. Four terrace planes (planes I-IV in order from the inland side) can be seen surrounding the island, which were formed by uplift during the Holocene. Aerial photographs show that the size of the reef fields differs from coast to coast. We began this study to investigate what factors are responsible for the differences in the size of the reef fields.
Methodology
Based on aerial photographs, we investigated the Holocene terrace III (formed between 4100 and 3100 years ago during the last 1000 years) at Onotsu, Shidoake, and Hanaraji, which are located on the east and west sides of the islands of different sizes. We set up a measuring line perpendicular to the coastline on Plane III and recorded the distance of Plane III. The number, size, type, and location of fossil coral assemblages found along the survey line were recorded. We also used a simple laser rangefinder to measure the distance and height difference between each point according to the terrace topography. The seafloor topography was taken from "Minna no Nautical Chart" and compared with the topographic map of the reef field.
Results
The topography of the terraces on the surface III was gentle in the order of Shido Oke, Hanaraji, and Onotsu. The size of the reef field was 50 m at Shido Oke, 43.5 m at Hanaraji, and 8 m at Onotsu with respect to the size of the Holocene terrace III (Figure 1). Compared to the present seafloor topography, the slope of the topography was more gentle at Shido Oke, Hanaraji, and Onotsu, in that order. The average sizes of the fossil coral assemblages at each site were 35.9 cm, 27.9 cm, and 16.7 cm in length at Shido Oke, Hanararo, and Onotsu, respectively. The size of the III surfaces was wider and larger corals were found in the order of the slope of the terrain being more gradual. The coral species were examined at Hanaraji, and out of the 86 colonies on the surveyed line, 67 were of the genus Green Lampreys and 13 were of the family Chrysophyllidae. Of the genus Greenishi, most of the species that could be identified were Yasuri Greenishi and Nioidori Greenishi. The frequency of green lanceolate and yellow lanceolate by length diameter showed that corals between 10 and 19 cm were the most frequent in both genera, and the overall frequency distribution was similar. As for the present seafloor topography, the slope of the seafloor topography and the slope of the reef plain tended to be the same.
4. Discussion
Reefs with a high percentage of corals with large diameter were wider than those with small diameter, suggesting a relationship between the diameter of corals and the size of the reefs. The similarity in the frequency of different coral species by diameter suggests that there is no difference in size distribution among coral species. The slope of both the topography and seafloor topography of the reef field was gentle in the wide area of the reef field and steep in the narrow area, suggesting that the slope of the seafloor topography is significantly related to the size of the reef field. The present bathymetry shows that the slopes of Shidoake and Hanaraji are more gradual, and therefore, it is expected that the reef field will expand more easily in the future.
5. Conclusion and Prospects
From the present experiment, it is considered that the size of the reef field is related to the seafloor topography and the size of the coral length. Considering the habit of corals to grow horizontally instead of vertically at depths shallower than 5 m, corals are expected to grow horizontally at shallower depths, and therefore, the seafloor topography is considered to be particularly relevant. In this presentation, we will also calculate and present the Coral Carbonate Production (CCP), which compares the amount of CO2 fixed by each reef plain.
In the future, we would like to increase the number of survey sites in order to confirm the relationship between the reef fields, corals, and seafloor topography that we have learned in this study. In addition to topography, we would also like to explore other factors such as the effect of wind, which we could not consider in this study.